Micromachined (MEMS) gyroscopes have become established as useful commercial items. Generally speaking, a MEMS gyroscope incorporates two high-performing MEMS devices, specifically a self-tuned resonator in the drive axis and a micro-acceleration sensor in the sensing axis. Gyroscope performance is very sensitive to such things as manufacturing variations, errors in packaging, driving, linear acceleration, and temperature, among other things. Basic principles of operation of angular-rate sensing gyroscopes are well understood and described in the prior art (e.g., Geen, J. et al., New iMEMS Angular-Rate-Sensing Gyroscope, Analog Devices, Inc., Analog Dialog 37-03 (2003), available at http://www.analog.com/library/analogDialogue/archives/37-03/gyro.html, which is hereby incorporated herein by reference in its entirety).
The principles of vibratory sensing angular rate gyroscopes with discrete masses are long-established (see, for example, Lyman, U.S. Pat. No. 2,309,853 and Lyman, U.S. Pat. No. 2,513,340, each of which is hereby incorporated herein by reference in its entirety). Generally speaking, a vibratory rate gyroscope works by oscillating a proof mass (also referred to herein as a “shuttle” or “resonator”). The oscillation is generated with a periodic force applied to a spring-mass-damper system at the resonant frequency. Operating at resonance allows the oscillation amplitude to be large relative to the force applied. When the gyroscope is rotated, Coriolis acceleration is generated on the oscillating proof mass in a direction orthogonal to both the driven oscillation and the rotation. The magnitude of Coriolis acceleration is proportional to both the velocity of the oscillating proof mass and the rotation rate. The resulting Coriolis acceleration can be measured by sensing the deflections of the proof mass. The electrical and mechanical structures used to sense such deflections of the proof mass are referred to generally as the accelerometer.
Many MEMS gyroscopes employ balanced comb drives of the type described generally in Tang, U.S. Pat. No. 5,025,346, which is hereby incorporated herein by reference in its entirety. General use of a micromachined layer above a semiconductor substrate with Coriolis sensing perpendicular to that substrate is described generally in Zabler, U.S. Pat. No. 5,275,047, which is hereby incorporated herein by reference in its entirety. Exemplary MEMS gyroscopes are described in Bernstein, U.S. Pat. No. 5,349,855; Dunn, U.S. Pat. No. 5,359,893; Geen, U.S. Pat. No. 5,635,640; Geen, U.S. Pat. No. 5,869,760; Zerbini, U.S. Pat. No. 6,370,954; and Geen U.S. Pat. No. 6,837,107, each of which is hereby incorporated herein by reference in its entirety. The latter four patents employ rotationally vibrated mass(es).
One problem in the manufacture of MEMS gyroscopes is that the Coriolis signals on which they depend are relatively small. It has been long recognized (e.g. Ljung, U.S. Pat. No. 4,884,446 or O'Brien, U.S. Pat. No. 5,392,650 or Clark, U.S. Pat. No. 5,992,233, each of which is hereby incorporated herein by reference in its entirety) that the signal size of a vibratory gyroscope can be magnified by operating the Coriolis accelerometer at resonance, i.e., by matching the frequencies of the accelerometer to that of the vibrating shuttle. Generally speaking, this increase in signal size eases the associated electronics requirements and thereby reduces cost. However, generally speaking, the larger the resonant amplification, the more sensitive is the accelerometer phase shift to small frequency perturbations. Such phase shifts are particularly deleterious to gyroscope performance, so it is generally necessary, in practice, to either well separate the frequencies or tightly servo the frequency of the accelerometer to the frequency of the shuttle. A mechanism for controlling the frequency of a differential capacitance accelerometer is conveniently available from changing the applied common mode voltage.
One technique for sensing the frequency match in order to close a servo loop around that mechanism is to apply a small, periodic perturbation to the mechanism and servo to zero response modulated at that periodicity. This is analogous to the mode matching servo commonly used in laser gyroscopes (e.g. Ljung, U.S. Pat. No. 4,267,478 or Curley, U.S. Pat. No. 4,755,057, each of which is hereby incorporated herein by reference in its entirety). This method uses the quadrature signal which directly couples from the shuttle and which can be separated from the useful, in-phase signal by synchronous demodulation. In practice, the magnitude of that signal generally varies widely and therefore is generally also subject to some control mechanism if the mode-matching servo is to have defined gain. This would be straightforward were it not that a real system generally has some other phase errors so that, for best gyro performance, the magnitude of quadrature signal should be near zero.
Another, method would be to apply a shuttle-frequency signal electromechanically to the accelerometer and synchronously demodulate the displacement output, servoing for the null which occurs at the 90 degree resonant phase shift. This inevitably interferes with the Coriolis signal and effectively is only applicable to those gyroscopes that do not need static response, such as camera stabilizing gyros.
The problem is addressed, at the expense of complexity, in Thomae, U.S. Pat. No. 6,654,424, which is hereby incorporated herein by reference in its entirety, by applying two such signals symmetrically disposed about the desired resonance and servoing for equality of response from them. This involves two signal generators, two demodulators, two filters and a differencing means, over twice the circuitry which one might otherwise expect for the servo.
In vibratory rate gyroscopes, numerous factors, such as imperfections in the various mechanical structures and in the electronics used for driving and sensing, can cause oscillations of the accelerometer that can be confused with Coriolis acceleration and rotation rate. Such error sources are often referred to collectively as gyroscope offset. There are two main classes of gyroscope offset, namely in-phase error and quadrature error. Generally speaking, quadrature error results when the vibratory motion is not perfectly orthogonal to the accelerometer. In the presence of quadrature error, the accelerometer experiences deflections proportional to the driven displacement. In-phase error results when the vibratory drive force is not perfectly orthogonal to the accelerometer. With in-phase error, the accelerometer experiences deflections proportional to the oscillation driving force which, at resonance, is also proportional to the oscillation velocity. Gyroscope offset can vary over time, for example, due to changes in temperature.
One possible approach to reducing gyroscope offset is to attempt to minimize the offset through device design, manufacture, and packaging, but there are practical limits to this approach.